Protein Molecular Weight Calculator
Estimate protein molecular weight from amino acid count or sequence length. Average MW per residue is 110 Da. Convert between Da, kDa, and compare to SDS-PAGE migration patterns.
🧪 Biochemistry📐 MW = n x 110 Da (approx)🧬 Biology
Number of amino acid residues
Adjust for signal peptide (residues removed)
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Protein Molecular Weight Calculator | — | MW = n x 110 Da (approx) | Da or kDa |
Step-by-Step Examples
Example 1
GFP Protein
GFP is 239 amino acids long.
- MW = 239 x 110 = 26,290 Da
- MW ≈ 26.3 kDa (actual: 26.9 kDa)
- Estimate within 3% of actual value
✓ MW ≈ 26.3 kDa (actual 26.9 kDa)
Example 2
Secreted Protein with Signal Peptide
Full sequence: 280 AA. Signal peptide: 22 AA.
- Net residues: 280 - 22 = 258 AA
- MW = 258 x 110 = 28,380 Da ≈ 28.4 kDa
- This is the mature secreted form
✓ Mature protein: 28.4 kDa
Example 3
Large Enzyme
Fatty acid synthase monomer is 2511 amino acids.
- MW = 2511 x 110 = 276,210 Da = 276.2 kDa
- Native FAS is a homodimer: 2 x 276 = 552 kDa
- Runs very high on SDS-PAGE or cannot enter 10% gel
✓ FAS monomer: 276 kDa, dimer: 552 kDa
Real-World Applications
SDS-PAGE Prediction
Estimate gel migration position before running experiment.
Molecular Cloning
Verify expressed protein size matches expected ORF translation product.
Antibody Selection
Choose antibodies optimized for target protein size range.
Bioinformatics
Cross-reference predicted MW with UniProt annotation and mass spectrometry results.
Common Mistakes to Avoid
⚠️
Using 110 Da as exact average
110 Da/residue is an approximation. Actual amino acid MWs range from 57 Da (Gly) to 186 Da (Trp). For accuracy, use ExPASy ProtParam with the actual sequence.
⚠️
Forgetting signal peptides
Signal peptides are cleaved during secretion. Use the mature sequence, not the full ORF translation, for expected size on gel.
⚠️
Ignoring post-translational modifications
Glycosylation adds 2-30+ kDa. Phosphorylation: +80 Da per site. Ubiquitination: +~8.5 kDa. These cause the protein to run higher than predicted.
Frequently Asked Questions
Why does my protein run at unexpected MW on SDS-PAGE? ▾
Possible reasons: glycosylation, other PTMs, protein aggregation, anomalous SDS binding (e.g., membrane proteins), or degradation products.
What is the average amino acid MW? ▾
Approximately 110 Da as residue (after water loss during peptide bond formation). Individual residues: Gly 57 Da, Ala 71 Da, Trp 186 Da.
How accurate is the 110 Da estimate? ▾
Within ±10% for most globular proteins. For proteins rich in heavy amino acids (e.g., Trp, Phe) the actual MW is higher.
What is the MW of common proteins? ▾
Insulin: 5.8 kDa, Lysozyme: 14.3 kDa, BSA: 66.4 kDa, IgG antibody: 150 kDa, Titin: 3,816 kDa (largest known protein).
How do I calculate exact MW? ▾
Use ExPASy ProtParam (web.expasy.org/protparam). Sum of residue masses + H2O (18.02 Da). Gives monoisotopic and average isotopic mass.